SEOUL, South Korea — North Korea on Saturday indefinitely postponed reunions of families separated by the 1950-53 Korean War scheduled to start Wednesday, an apparent setback after weeks of improving ties that followed springtime threats of war.

North Korea said the six days of reunions, which last happened three years ago, could not be held because of South Korean conservatives’ “reckless and vicious confrontation racket” against Pyongyang.

It also vowed, in familiar rhetoric, to “take strong and decisive counteractions against the South Korean puppet regime’s ever-escalating war provocations.”

The development, which an analyst called a North Korean attempt to gain an advantage in negotiations with Seoul, is a twist in what had been easing tensions on the Korean Peninsula, with Pyongyang tempering its threats and pursuing talks meant to restart various inter-Korean cooperation projects.

The biggest highlight is the recent return of North and South Koreans to a jointly run factory park just across the border in North Korea.

South Korea’s Ministry of Unification, which is responsible for ties with the North, denounced the decision to postpone the reunions for political reasons as inhumane and unacceptable. A South Korean delegation responsible for preparing for the reunions is currently in the North, the ministry said.